stbt run: Save screenshot for any Exception object with a screenshot
stbt run: Save screenshot for any Exception object with a screenshot

...not just MatchTimeout exceptions.

This means that `stbt run` now saves the screenshot for `MotionTimeout`
exceptions; also, script writers can define their own Exceptions
(perhaps subclasses of UITestFailure) and attach screenshots to them.

martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester